1. Field of the Invention
This invention relates to polymerization processes and, more particularly, it relates to the formation of particulate polymers of the type which are insoluble in a liquid monomeric medium in which the polymerization is effected. Still more particularly, this invention relates to a novel method for the bulk polymerization of vinyl halide, and particularly the polymerization of vinyl chloride.
2. Description of the Prior Art
Various polymerization processes are known in the art wherein a polymer is formed in a polymerization atmosphere wherein the monomer, or mixture of polymerizable monomers, is maintained as a liquid with the resultant formation of a polymer which is insoluble in the liquid monomeric system. The bulk polymerization of vinyl halides, including the bulk polymerization of the vinyl chloride homopolymer, as well as the bulk polymerization of copolymers of vinyl chloride with other compatible copolymerizable monomers, is well known in the art. In the bulk polymerization of vinyl halides, for example, vinyl chloride, a monomer is typically put in a suitable polymerization reactor, so as to maintain it in a liquid condition, and in the presence of an effective polymerizing amount of a free radical catalyst or initiator and under temperature and pressure conditions favorable to polymerization, the resultant polymer is formed which is insoluble in the monomer system, the monomer, of course, being depleted as polymerization proceeds. The polymerization may, of course, proceed to a suitably selected end point, at which point the polymerization, even if not entirely complete, may be stopped and the product isolated.
While those skilled in the art realize that relative to other polymerization processes, the product of a bulk polymerization process is generally substantially free of impurities and the process for producing this product is potentially efficient and economical, these benefits, especially the latter two, have not been fully achieved in the area of bulk vinyl polymerization. Thus, for example, a simple and economical bulk polymerization process for the formation of vinyl polymers, for example, vinyl halides and especially vinyl chloride homopolymers, and copolymers of vinyl halides with compatible copolymerizable monoethylenically unsaturated monomers, has not yet become a commercial reality. The reason for this apparently resides in the fact that these types of bulk polymerization reactions are highly exothermic and with the free radical initiators employed proceed at a rate which is strongly dependent on temperature. Because of the foregoing characteristics of the reaction, along with the fact that severe problems with heat transfer are encountered in the reaction because of the viscosity increase in the system, these bulk polymerization processes have been very difficult to control. This lack of control creates special problems in the bulk polymerization of vinyl chloride, because of the difficulty of maintaining good heat transfer during the entire polymerization reaction wherein the reaction mass is a thick slurry at a conversion of about 15-20%, then proceeds to a wet cake at a conversion of about 25-30% and a powder after a conversion of about 45-50%. These control problems are obviously not conducive to the formation of a high quality product inasmuch as, for example, extensive localized heating may cause the formation of significant excessive amounts of skins and large thermally degraded aggregates which, of course, are undesirable. Poor process control also results in the formation of gels, or fisheyes, in the final product.
Attempts have been made in the art to resolve some of these difficulties but they typically employ the use of a plurality of reactors to accomplish the total polymerization and also require the use of different agitator speeds during the polymerization process with the initial agitation speed being quite high and thereby consuming large amounts of power. Thus, these attempts are not economically satisfactory in that they require substantial initial capital investments and the operating costs are also high.
A typical example of a heretofore practiced prior art procedure for overcoming the difficulties inherent in the bulk polymerization of vinyl chloride is set forth in U.S. Pat. No. 3,522,227. In this patent, a two stage bulk polymerization process is set forth wherein polymerization of vinyl chloride is carried out in a first stage under severe conditions of agitation and high turbulence and thereafter completed in a second stage which is preferably a different reactor maintained under much milder conditions of agitation. It should become immediately apparent that this prior art procedure resulted in an extremely high cost in the first stage with respect to equipment and operating power in order to carry out the high speed agitation. The high speed was thought to be necessary in order to overcome the difficulties which were encountered in the bulk polymerization of vinyl chloride such as, for example, production of a particulate product.
A far more effective solution to the overall problem is set forth in copending application Ser. No. 111,496, filed Feb. 1, 1971. This application is directed towards the use of a frusto-conical type reactor wherein the bulk polymerization of vinyl chloride can be carried out in an efficient manner without necessitating the high speed agitation which the prior art workers thought necessary. However, the polymeric product resulting from the process of said application, although admirably suited for a wide variety of applications, nevertheless, did appear to cause some problems when said material was desired to be used in the formulation of plastic bottles.
For reasons which are not completely understood, it appears that in the fabrication of plastic bottles from polyvinyl chloride powders, there is a certain broad range of desired particle size which makes the polymeric product compatible with the processing equipment necessary to produce these bottles. The particle size distribution which was obtained by the process of Ser. No. 111,496 was not optimum for the production of plastic bottles.
Additionally, it is noted that in all the prior art processes for the bulk polymerization of vinyl chloride there is an inherent disadvantage in that a substantial amount of polymer in the form of a scale is deposited on the internal surfaces of the reaction vessel which builds up during the course of the polymerization reaction up to the point where it must be removed. The removal of this material is expensive not only from the point of view of the manpower which is required to scrape or otherwise remove the material from the internal surfaces of the equipment, but also due to the fact that there is down time while the procedure is being carried out thereby resulting in an economic liability. A further economic penalty arises from the fact that the scale must be mechanically ground and then sold as a "second grade" product.
The process of this invention admirably overcomes the difficulties of the heretofore practiced processes. In the first place, the particle size distribution of resulting polyvinyl chloride powders are well suited for the production of plastic bottles. Secondly, for reasons which are not completely understood, it has been found that in the present process, there is substantially less build-up of polymeric material on the internal surfaces of the reactor, thereby directly minimizing the effort which is necessary to remove the same and accordingly increasing the economic viability of said process. Additionally, the process of this invention minimizes the amount of extremely fine powder which is produced in the polymeric product. As can well be appreciated, fine powder is an economic liability due to the fact that it presents a potential health hazard in that it cannot be breathed into the lungs so that expensive filtering equipment must be used in order to prevent it from entering into the atmosphere. Additionally, fine powder is extremely abrasive and can cause damage to the various equipment with which it comes in contact.